Saturday, June 22, 2002

Wandering uranium...

Kathy Kinsley notes the discovery of 2 kg of uranium in Russia in a company car, as reported in the Hindustani Times. Is this a big deal?

IMO somebody wanted something scary to write. For one, why 2000 microRoentgen, instead of 2 milliRoentgen or .002 Roentgen? Physically speaking, if a piece of pure uranium of this mass were in the form of a cube, it would be less than 2" to a side.

Warning - I am not a health physicist. But I worked with them for a few years at commercial nuclear power plants. I was always interested in my exposure for obvious reasons, so I learned some interesting things. And I'll give you links to what I'm talking about, so take it for what you're paying for it.

The first thing to note is that there is energy exposure, and energy absorption. Your microwave oven provides an example. It puts out microwave energy, but this energy isn't absorbed by everything - it's designed to be absorbed by water. Then the water molecules heat up and transfer that heat to the rest of the food. It won't do to try to use a microwave oven to try to heat something without water in it (or some other substance that will soak up microwaves). Similar logic applies to sunscreens - they don't affect the sunlight, just how it interacts with your skin.

Likewise what matters about the radiation is how much of it is absorbed by the body, not how much is there. That's why we have units like roentgens (R) for energy, radiation absorbed dose (Rads), and radiation equivalent in man (rem, also sieverts). The first is about how much energy is there, the second is about how much of it is absorbed, and the third is a measure of the impact.

Measure of the impact? Yes. This is necessary because not all parts of your body are equally sensitive - the quicker the cells grow, the more susceptible they are to radiation. That's why radiation picks on cancer cells, and why federal law requires us to chase pregnant women out of situations where they might get significant radiation dose. Your outer skin surface is already dead, so radiation there means nothing. Your lower extremities have no vital organs, so radiation matters less there. Your bone marrow is very susceptible, so the radiation equivalent is higher than for your lower extremities.

From here, "Rem: (Roentgen Equivalent Man) A unit used to express all types of ionizing radiations on a common scale to indicate relative biological effects. For beta and gamma radiations: Exposure to 1 Roentgen delivers a dose of 1 Rad, which is equivalent to 1 Rem." This is not strictly true in that the same dose to your hands isn't the same as that to your bone marrow (which is why we speak of equivalents), but it's the most comprehensible thing I found on the Web in a few minutes of looking.

But...2000 microRoentgen per hour as cited in KK's post then equates to 2000 microrem per hour or 2 millirem per hour. OK, that's 48 mrem per day. The historical dose rate cited in KK's post is .2R/day, which would convert to 200 mrem per day per the earlier conversion (with caveats as noted). This would imply a permissible yearly dose of 73000 mrem, as compared to a federal maximum of 5000 per year when I last looked. In practice 100 mrem per day is a common daily limit, and we would aim for no more than 1000mrem/year.

Again, but...not all radiation has the same properties. For everyday purposes there is alpha, beta and gamma radiation. With gamma there's nowhere to run and nowhere to hide - it'll penetrate anything. OTOH alpha won't even penetrate your skin, and it doesn't take much to block beta (gloves and glasses mostly). But when it hits something vital alpha will do the most damage, beta less so, and gamma the least. The outside layer of your skin is already dead, and because it repulses the alphas, the real impact of external alpha radiation is almost nil. And uranium is an alpha emitter.

There will be some gamma present for reasons beyond the scope of this post. But the idea is that this occurrence isn't exactly a radiological disaster in itself. I'm awfully interested in what it's doing where it was found though...

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